In recent years, pressurized fluidized bed combustion of coal has interested utilities worldwide for the generation of electric power in an environmentally acceptable manner. In such facilities, ambient temperature coal and limestone are fed by means of lockhoppers into a fluidized bed combustor operating at pressures of about 40 pounds per square inch to about 225 pounds per square inch or more, and at temperatures of between about 1600 to 1830.degree. F. (about 870.degree. to 1000.degree. C.), in which 90 percent of the oxides of sulfur and nitrogen present in the coal are absorbed by the limestone. The gaseous combustion products are cleaned of larger particulates and then let down to atmospheric pressure through an expansion turbine which drives an air compressor and possibly an electric generator. Water boils inside tubes that are present in the combustor to provide steam that is also used to generate electric power.
This cycle has resulted in the construction of several prototype units in the United States, England, Sweden, and elsewhere. A plant in England, at Grimethorpe in South Yorkshire, has been in operation since 1982. A persistent problem with that operation has been the removal of hot, pressurized limestone-ash mixture from the combustor and from the associated gas cleaning or filtering equipment, all of which operate at combustor pressure and temperature conditions. Initially, the intent was to remove the limestone-ash mixture by means of batch water slurry tanks. The solids would be dumped into an agitated tank at system pressure and partially filled with water, where they would be quench cooled to below 250.degree. F. (about 120.degree. C.). The tank would then be isolated from the pressurized system, depressurized, and emptied of the solids and water slurry formed. Problems arose, however, with ash sticking, like concrete accretions in the tanks, erosion of the tanks, and water erupting up into the combusition system equipment.
Problems relative to filtering of the hot combustion gases also exist, as discussed in co-pending application Ser. No. 013,300, filed Feb. 10, 1987, in the names of Gordon Israelson, et al. (W.E. 53,607), and assigned to the assignee of the present invention, the contents of said application incorporated by reference herein. Hot solids collected in the filter system, which is pressurized and at high temperature, are collected in conical-shaped sections of a filter housing and discharged through a solids removal device such as a lockhopper or a screw conveyor, or the like.
It has been proposed to use high temperature alloy valves and refractory lined tanks to function as a lockhopper system to depressurize the discharged ash solids in a dry state, and then cool the ash solids for disposal. The valves, however, were expensive. Also, subsequent test work sponsored in the United States by EPRI showed that the high temperature fine solids exhibited a sticky property, so that the dry solids would not flow by gravity through the lockhopper system at temperatures above about 1200.degree. F. (650.degree. C.). The sticky property of the ash and limestone fine solids seemed to be related to the high temperature and possibly to the presence in the solids interstices of a gas containing water vapor with traces of sulfur dioxide and trioxide. Fly ash carried overhead from a pressurized fluidized bed combustor and collected on a filter has a tendency to clump up and stick together until cooled to a temperature well below 1600.degree. F. (about 870.degree. C.) at which the filter is operated. Cooling of the ash to below about 1000.degree. F. (about 540.degree. C.) removes the clumping tendency such that the ash will flow freely. The presence of carbon dioxide did not promote clumping, but it is conjectured that steam, sulfur dioxide and sulfur trioxide gases present may have an effect on the stickiness of the hot ash.
It is an object of the present invention to provide a dry discharge system for hot ash solids from a coal combustor or associated filter that avoids the problems of previous such discharge systems.